Non-obtrusive, real-time physiological monitoring would be beneficial in civilian health care settings, in athletes, and operationally in military or civilian service members. Knowing an individual's physiological status can lead to targeted preventative interventions, improved performance, risk mitigation in dangerous contexts and immediate monitoring following injury or illness. Current physiological monitoring systems are capable of measuring a host of useful physiological parameters such as heart rate (HR), core temperature (Tc), and breathing rate (BR); however, these systems suffer from limited operational utility due to ergonomic obtrusiveness (wearing a harness, for instance), limited battery life, and poor signal fidelity during movement.
HR and BR are essential vital signs used in the evaluation of the physiologic status of children and adults in clinical and non-clinical settings. They constitute the initial measurements in acutely ill patients and provide the basis for clinical severity stratification as well as markers of response to life-saving cardiopulmonary resuscitation. Additionally, HR and BR serve as non-diagnostic indicators of performance status in service members and in performance athletes.
There are numerous methods for monitoring HR and BR, but most require the attachment of superficial sensors to the body. HR can be monitored using electrical methods such as electrocardiogram (ECG), optical methods such as photoplethysmography (PPG) (pulse oximetry), or mechanical methods such as ballistocardiography. BR can be monitored directly using trans-thoracic plethysmography and expired gas analysis approaches, or indirectly using advanced processing methods such as PPG. All of these methods have some limitations, as they may cause patient discomfort, skin irritation, and they cannot be used reliably in high-physical activity contexts where motion artifacts may corrupt the signal. Over the last decade, ingestible medical devices have gained broad acceptance; for example, ingestible devices can measure pH, and video capsule endoscopy is widely used for diagnosis of gastrointestinal (GI) pathology. Vital sign monitoring from within the GI tract may be a safe and effective alternative to existing clinical monitoring systems.